Restratification Structure and Processes in the Irminger Sea

Abstract

The Irminger Sea is one of the few regions in the global ocean where deep convection occurs. Deep convection is followed by restratification during summer, when the stratification of the water column is reestablished and the convectively formed dense water is exported to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). We investigate the interannual variability and physical drivers of restratification in the upper 600 m of the Irminger Sea using the CMEMS oceanic reanalysis data for the years 1993--2019. We find that there are distinctly different restratification processes in the upper 100 m of the water column (the upper layer) and the water below it (the lower layer). In the upper layer, the stratification is dominated by the strong seasonal cycle. Restratification starts when the surface heat flux becomes positive, and ends when the heat flux becomes negative. The minimum stratification in winter is always zero, because the mixed layer always extends below the bottom of the upper layer. Interannual variability in the maximum summer stratification is mostly caused by warm and fresh anomalies that cause high stratification, and cold and saline anomalies that cause low stratification. In 2010 and 2019, there were peaks in stratification in the upper layer, which could be related to the combination of surface warming and freshwater gain. In 2010 meltwater from Greenland likely contributed to the high freshwater anomaly. In 2019 an extreme freshwater anomaly circulating through the subpolar North Atlantic added to the freshwater anomaly. By contrast, in the lower layer the seasonal cycle is weaker and there is strong interannual variability. Restratification can continue for up to 5 months after the surface heat flux has become negative, which indicates a role for lateral advection. The maximum summer stratification in this layer is determined by the minimum stratification in the preceding winter as well as the strength of restratification during summer. The minimum winter stratification is related to the mixed layer depth in that winter. The strength of restratification is strongly correlated with eddy kinetic energy in the eastern Irminger Sea. This suggests the lateral advection is driven by warm, saline eddies shed by the Irminger Current. In the future, increased ocean surface temperatures and meltwater from the Greenland Ice Sheet and Arctic sea ice due to anthropogenic climate change are expected to increase upper layer stratification in the Irminger Sea, potentially inhibiting mixing and thereby slowing down deep convection.